We have identified a secreted orthopoxvirus-encoded protein, termed OMCP, which binds two receptors on innate immune effector cells. The first is NKG2D - an activating receptor of NK and T cells. The second, as yet undefined, is expressed on monocytes/macrophages and B cells. These binding activities are conserved in humans and all tested rodent species. We hypothesize that OMCP is secreted by orthopoxvirus-parasitized cells to competitively antagonize both NKG2D and the macrophage receptor. This prevents each receptor from binding its respective ligand and triggering immune function against the infected cell. We further hypothesize that actions of OMCP allow these pathogens to reach higher titers in infected tissues, particularly mucosal surfaces, prior to the onset of adaptive immunity. Our objectives are: [1] to understand the biophysical details of how this happens including the thermodynamic binding parameters and atomic-scale structures of each involved protein;and [2] to establish why blockade of these receptors enhances viral fitness. Accordingly, we will identify and clone the macrophage OMCP receptor, produce recombinant proteins, and characterize the respective molecular interactions using binding assays and X-ray crystallography. We will also generate recombinant viruses expressing a null OMCP allele as well as viruses expressing monospecific OMCPs so that the effects of each binding activity can be studied in isolation. The relevance of this work to public health stems from the threat posed by emerging rodent-borne orthopoxviruses that occasionally cause severe disease in people. Unlike smallpox, these viruses cannot be eradicated by vaccination;their animal vectors have ongoing contact with humans;and they are accessible in the wild to malicious entities for engineered acquisition of immunoevasive and/or drug- resistant phenotypes. Surprisingly little is known about how the immune system detects these agents, and how they in turn counter that detection. Study of OMCP will inform this issue since it binds to two molecules that, almost by definition, are important to these viruses'life cycles in their hosts. This work will also provide immunologic insight into the function of these receptors insight that likely is extensible to other infections.